The present invention relates generally to electrically operated power tools and in particular, to power tools that are powered by a brushless DC motor.
Over the past couple of decades the use of cordless power tools has increased dramatically. Cordless power tools provide the ease of a power assisted tool with the convenience of cordless operation. Generally, cordless tools are driven by a Permanent Magnet (PM) brushed motor that receives DC power from a battery assembly or converted AC power. The motor associated with a cordless tool has a direct impact on many of the operating characteristics of the tool, such as output torque, time duration of operation between charges and durability of the tool. The torque output relates to the capability of the power tool to operate under greater loads without stalling. The time duration of the power tool operation is strongly affected by the energy efficiency of the motor. Since, during some operating modes cordless tools are powered by battery modules that contain a limited amount of energy, the greater the energy efficiency of the motor, the longer the time duration that the tool can be operated. The durability of a power tool is affected by many factors, including the type of motor that is used to convert electrical power into mechanical power. Brushed motors such as the PM brushed motors that are generally employed in power tools are susceptible to damaged brushes during rough handling.
Conventional permanent magnet brushless DC motors provide an ineconomical alternative to brushed DC motors. Although brushless DC motors generally are more durable and provide higher speed and torque performance than similar size brushed motors, conventional brushless motors have daunting cost disadvantages. Before expanding on the cost disadvantages of brushless DC motors, first an overview of the operating characteristics of the two types of motors will be presented.
The main mechanical characteristic that separates Permanent Magnet brushless motors from Permanent Magnet brushed motors, is the method of commutation. In a PM brushed motor, commutation is achieved mechanically by means of a commutator and brush system. Whereas, in a brushless DC motor commutation is achieved electronically by controlling the flow of current to the stator windings. A brushless DC motor is comprised of a rotor for providing rotational energy and a stator for supplying a magnetic field that drives the rotor. Comprising the rotor is a shaft supported by a bearing set on each end and encircled by a permanent magnet (PM) that generates a magnetic field. The stator core mounts around the rotor maintaining an air-gap at all points except for the bearing set interface. Included in the air-gap are sets of stator windings that are typically connected in either a three-phase wye or delta configuration. Each of the windings is oriented such that it lies parallel to the rotor shaft. Power devices such as MOSFETs are connected in series with each winding to enable power to be selectively applied. When power is applied to a winding, the resulting current in the winding generates a magnetic field that couples to the rotor. The magnetic field associated with the PM in the rotor assembly attempts to align itself with the stator generated magnetic field resulting in rotational movement of the rotor. A control circuit sequentially activates the individual stator coils so that the PM attached to the rotor continuously chases the advancing magnetic field generated by the stator windings. Proper timing of the commutation sequence is maintained by monitoring sensors mounted on the rotor shaft or detecting magnetic field peaks or nulls associated with the PM.
Generally, existing brushless DC motors that provide a specified power output within a volume that is appropriate for portable power tools are too costly for the consumer market (by a factor of 10). The most significant factors driving the cost of a brushless DC motor are the power density, the cost of the permanent magnets and electronic components, and complex production procedures. Therefore, to reduce the cost of producing brushless DC motors either the cost of the permanent magnets must be reduced, the method of assembling the devices must be improved, or the power density must be increased. The cost of the permanent magnets can be reduced by using either smaller or less powerful permanent magnets. The power density of a brushless DC motor can be increased by using higher power PMs or reducing the resistance of the stator windings.
The present invention provides a system and method for reducing the cost of producing brushless DC motors. The brushless DC motor includes a rotor assembly that has an unmagnetized permanent magnet affixed to a shaft. The permanent magnet remains unmagnetized until the motor is partially assembled. A plurality of coils for producing a magnetic field are wound about the rotor assembly. The coils include end turns that enclose the rotor assembly such that the rotor assembly is not removable. Since the windings are wound with the rotor assembly already enclosed, the windings do not require large end coils to allow subsequent insertion of the rotor. Minimizing the end coils reduces the length of wire required per turn, thereby reducing the resistance of the winding. Also, since the PMs are unmagnetized when the coils are wound around the rotor assembly the winding process is simplified by not coupling energy into the wire which would interference with the winder operation. In addition, enclosing the rotor assembly with the coils improves the coupling between the permanent magnet and the coils, thereby permitting the use of a smaller permanent magnet. The wound assembly is inserted into a stator stack comprised of ferrous material that provides a magnetic flux return path for the magnetic flux generated by the PM and stator windings. Using an unmagnetized PM facilitates easy insertion of the wound assembly into the stator stack, reduces the accumulation of metallic debris during the manufacturing process, and permits the motor assembly to be sealed prior to magnetizing the PM.
For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings.